This invention relates to a novel ethylene (co)polymer, a blow molding process in which thickness of parison is adjusted by using the (co)polymer, a method of improving fire resistance of moldings and laminate and hollow molding or blow molded products made by using the same. More particularly, this invention relates to an ethylene (co)polymer which is excellent in mechanical strengths such as impact resistance, moldability, etc. and applicable to extrusion molding, injection molding, blow molding, etc., which is excellent for use in large blow molded products required of drawdown resistance, puncture resistance, parison thickness controllability (parison control response), pinch-off properties, durability, etc., in particular excellent in fire resistance required for fuel tanks, to a method for freely adjusting thickness of parison by using the (co)polymer, to a method of improving fire resistance of moldings and to hollow moldings, laminates and hollow multilayer moldings made by using the same, which exhibit superiority in durability and fire resistance.
Generally, detergent bottles, food containers such as beverage bottles and vegetable oil bottles, large vessels such as drum cans and industrial cans, fuel containers such as kerosene cans and gasoline tanks, spoilers, bumpers and the like are produced by a blow molding process. The blow molding process is to pinch with a mold a cylindrically extruded parison composed of a melted resin, blowing air into the parison to blow up and change the shape of it so as to conform to the shape of the cavity of the mold, and then cooling it.
The blow molding process is widely utilized since it is applicable widely to large fuel tanks having complicated shapes, drums, and further panel-like moldings as well as hollow moldings such as bottles and since molding is simple and molding costs covering molds are inexpensive.
Recently, in the field of large vessels such as drum cans and large containers, substitution by plastics is being carried forward positively taking into consideration freedom in shape, economy, safety, and adaptability to environmental problems.
It is also the case in the field of automobile industry such as fuel tanks, bumpers and spoilers. Particularly in the field of fuel tanks, substitution by plastics proceeds remarkably along with the development of fuel permeation preventing technology by a co-extrusion technology, a sealer technology, a continuous multi-ply technology, etc.
When large products are blow molded, a phenomenon that a parison droops down due to its own weight (drawdown) tends to occur. To make the drawdown small, it has been known to use a resin having sufficiently high viscosity and melt tension.
In molding hollow moldings having a complicated shape, there tends to occur a local increase in blow ratio to cause puncture or local thinning of a parison. To prevent the puncture or local thinning of a parison, it has been known to control the thickness of the parison (parison control) to make a desired portion thick or make alterations in the appliance such as alteration of the shape of a mold.
Conventionally, there has been a problem that an increase in molecular weight and an increase in viscosity in order to improve the drawdown resistance of polyethylene results in aggravation of extrusion properties (extrusion rate, parison surface conditions) and welded strength of a parison becomes worse so that the pinch-off shape is worsened. As a method for solving the problem have been known, for example, the method in which a multi-step polymerization method using a Ziegler-type catalyst (Japanese Patent Application Laid-open No. Sho 55-152735), the method in which a small amount of radical generator and crosslinking auxiliary are added to polyethylene resin (Japanese Patent Publication Hei 2-52654), the method in which two polyethylene components are blended in a certain proportion (Japanese Patent Application Laid-open No. Hei 6-299009), etc.
However, recently larger and more complicated blow moldings are being produced so that further improvement of drawdown resistance of a resin is becoming necessary in order to extrude a heavy parison to a larger length in a stable manner. Also, in order to improve the fuel permeation of a fuel tank, there is a demand for a multilayer fuel tank provided with an ethylene/vinyl acetate copolymer saponification product (hereafter, called EVOH), which has poor heat resistance, as a barrier material. In molding them, a blow molding machine of a continuous extrusion type having a smaller retention portion and suffering less thermal decomposition of EVOH is becoming to be put in use for blow molding larger fuel tanks in place of a blow molding machine of an accumulator type which has heretofore been used for larger size molding. As compared with the accumulator type, the continuous extrusion type molding machine takes a long time for extruding a parison so that drawdown of a parison tends to occur more frequently. Therefore, there is a demand for a resin having an increased drawdown resistance.
For this purpose, the improved polyethylenes obtained, for example, by the above-mentioned processes are insufficient in drawdown resistance for obtaining large hollow moldings.
Also, according as the shape of hollow moldings becomes more complicated, there come into question phenomena that parisons will puncture when they are being formed by blowing and that local thinning of a product will occur at its corner portions, etc.
To prevent these problems, a method has generally been used in which a parison controller is used in order to control the thickness of a parison. However, conventional polyethylene and resin compositions thereof grow to have insufficient parison thickness controllability (parison control response) so that improvement of parison control response is desired. Also, as the method for enabling molding of deep drawn articles having complicated shapes, there have been known, for example, the method in which a split mold is used (Plastics, Vol. 42, No. 5, p.64-71) and the method in which a blow molding machine with a mold slanting mechanism is used (Plastics, Vol. 41, No. 10, p.59-69). However, the above-mentioned methods have a problem that costs for the apparatus and mold are high.
Furthermore, in the field of fuel tanks, which require fire resistance as one of the requisite properties, there have been proposed ethylene copolymers having an xcex1-olefin content within a specified range and inherent viscosity, zero shear viscosity and time for melt stretching breakage being prescribed to be within respective specified ranges as a preferred material (Japanese Patent Application Laid-open No. Hei 7-101433). However, they are in an insufficient state in molding processability and fire resistance to cope with weight reduction and thickness reduction.
An object of the present invention is to overcome the defects of conventional polyethylenes and provide an ethylene (co)polymer having excellent drawdown resistance, puncture resistance, parison control response, and pinch-off properties.
Another object of the present invention is to provide a method for freely adjusting thickness of a parison by using the above-mentioned ethylene (co)polymer or composition thereof and blow molded products which are improved against the phenomena that the parison of blow molding having a complicated shape will puncture when expansion shaped and that there will occur local thinning of corner portions of products by using the said method. Furthermore, it is to provide blow molded products which are excellent in fire resistance by use of the above-mentioned ethylene (co)polymer or its composition.
Still another object of the present invention is to provide blow molded multilayer containers having a capability of preventing permeation of various fuels and causing no local thinning.
In a first aspect, the present invention relates to an ethylene (co)polymer which satisfies the following requirements (a) to (d):
(a) a density of 0.93 to 0.98 g/cm3,
(b) a molecular weight distribution (Mw/Mn) of 25 to 50,
(c) that values of slope (A) and of intercept (B) obtained from a die swell (DS) and a shear rate ({dot over (xcex3)}) measured in a region of shear rate ({dot over (xcex3)}) (6.08 to 24.8 secxe2x88x921) at 230xc2x0 C., in accordance with the equation (1) below
DS=Axc3x97ln({dot over (xcex3)})+Bxe2x80x83xe2x80x83(1)
are such that 0xe2x89xa6Axe2x89xa60.05 and 1.25xe2x89xa6Bxe2x89xa61.45,
(d) that the stress ratio of a stress ("sgr") at a strain of 1.0 to a maximum stress ("sgr"max) in a stress-strain curve for uniaxial stretching at 0.1 secxe2x88x921 measured at 170xc2x0 C. is such that "sgr"max/"sgr"xe2x89xa72.1.
Preferably, the present invention relates to an ethylene (co)polymer which further satisfies the requirement (e) that a value of xcex1 as defined by the equation (2) below
xcex1=xcex5/txe2x80x83xe2x80x83(2)
(wherein xcex1 is xcex5/t when xcex5 defined by xcex5=-ln(sample diameter after t seconds It/initial sample diameter Io) is 0.6) when uniaxially stretched under a constant tension such that initial stress (=tension/initial cross section of sample) measured at 170xc2x0 C. is 3.0xc3x97104 Pa is not greater than 0.03 and time for sample breakage is not shorter than 25 seconds.
More preferably, the present invention relates to an ethylene (co)polymer which further satisfies the requirement (f) HLMFRxe2x89xa610 g/10 min and which has a C3-20 xcex1-olefin content of 10 mol % or less.
In a second aspect, the present invention relates to blow molded products comprising the above-mentioned ethylene (co)polymer or composition thereof, preferably a fuel tank which exhibits superiority of the above-mentioned ethylene (co)polymer or composition thereof.
In a third aspect, the present invention relates to a laminate comprising a layer comprising the above-mentioned ethylene (co)polymer or composition thereof, a barrier layer, and optionally an adhesive layer and preferably to a laminate in which the barrier layer comprises at least one member selected from polyamide resins, ethylene/vinyl acetate copolymer saponification products, polyester resins, polyvinylidene chloride resins and compositions thereof. It is desirable that for the adhesive layer, adhesive resins which comprise copolymers of unsaturated carboxylic acids or derivatives thereof with olefins or polyolefin resins modified with unsaturated carboxylic acids or derivatives thereof be used.
In a fourth aspect, the present invention relates to a blow molded multilayer container comprising a laminate which comprises a layer comprising the above-mentioned ethylene (co)polymer or compositions thereof, an adhesive layer and a barrier layer, and preferably such exhibits superiority in multilayer blow molded fuel tanks.
In a fifth aspect, the present invention relates to a blow molded multilayer container comprising a laminate which comprises at least one resin layer selected from the recycled resin composed of the above-mentioned laminate, or compositions of the ethylene (co)polymer according to the first aspect containing the recycled resin, or compositions of these with polyolefin resin, a barrier layer, and optionally an adhesive layer, and preferably such exhibits superiority in physical properties and economy in blow molded multilayer fuel tanks.
In a sixth aspect, the present invention relates to a blow molding process in which thickness of a parison is adjusted by using the above-mentioned ethylene (co)polymer satisfying requirements (a) to (d) or composition thereof, changing the die gap, under the condition that the extrusion volume is substantially constant. The thickness of the parison having a complicated shape can be controlled easily by the said process, the parison will not puncture when expansion shaped, local thinning of corner portions will be improved, and in particular this process exhibits superiority in molding of large blow molded products having a complicated shape.
In a seventh aspect, the present invention relates to a method of improving fire resistance of blow molded products comprising the above-mentioned ethylene (co)polymer satisfying requirements (a) to (e) or composition thereof, and especially this method exhibits superiority in fuel tanks.